During the smelting of aluminum scrap such as aluminum house siding, castings, beverage cans and the like to recover aluminum therefrom, large quantities of salt fluxing compounds comprising sodium and potassium chlorides and fluorides, are added to the smelting furnaces. Such compounds are also added to furnaces employed during the smelting of aluminum dross during the process of recovering additional aluminum values therefrom. Such fluxing compounds combine with the aluminum oxide present in the furnace melt, and being lighter than the molten aluminum float to the top of the melt, forming a dross which is readily removed from the melt by skimming.
Inasmuch as the dross contains a number of valuable components, including but not limited to aluminum, and further, in view of the fact that the disposal of the dross in an environmentally responsible fashion entails undersirable expense, it has become advantageous to treat the dross, and to recover resusable components therefrom to the extent possible. Such a procedure not only salvages valuable materials, but importantly, the amount of material that must be disposed of, for example in an expensive landfill operation, is dramatically reduced.
In the past, a number of methods have been proposed and utilized for treating aluminum dross, some of them being relative primitive. One such procedure, for example, simply involves crushing and concentrating the dross in a suitable impact mill and melting the fragmented concentrate in a furnance where the aluminum present is freed from the dross as molten metal, the balance being discarded.
A still different method entails crushing the dross and leaching the fragmented material on a continuous basis to recapture the fluxing salts contained therein, the insoluble balance of the material being furnaced as previously described.
In some instances the dross has even been disposed of in a landfill without any attempt to recover contained reusable components including, for example, the sodium and potassium chloride "salt cake", one of the products discharged from the smelting furnances.
A more sophisticated method of dross treatment involves the method described in co-pending application Ser. No. 451,298, filed Dec. 15, 1989, entitled "Process for the Recovery of Values from Secondary Aluminum Dross", and continuation applications based thereon, directly or indirectly in the process there disclosed, after being crushed, the reduced particles are screened, the fines being sent to a screw classifier where they are digested with water. Solids remaining after classification treatment are dried, thereby transforming them into a useful product. Water from the classifier is processed to recover the flux salts therefrom.
The coarse material from the screening process is sent to a digester, treated with water, with thereafter wet screened. The insolubles from the screening are fed to a dryer and separated into coarse solids or "concentrate" which is sent to a furance for removal of aluminum as molten metal. The fine material is combined with solids discharged from the dryer used to process material leaving the screw classifier previously referred to.
The soluble-rich, semi-liquid stream leaving the wet screening operation is also treated in a screw classifier; the solids therefrom being combined with the dryer feed from the first-mentioned classifier. The liquids leaving the two classifiers are also combined, and the fluxing salts removed therefrom. While the process described offers considerable advantages over the other methods, outlined, it is not without certain disadvantages.
In this regard, it has been found, for example, that in the course of digestion of the coarse fragments produced in the crushing operation, especially the digestion of beverage can dross including associated salt cake, a number of underisable and potentially and potentially hazardous reactions take place in the presence of water. For instance, some of the aluminum compounds unavoidably present in the dross combine to form dangerous and noxious products. For example, free aluminum in the dross can combine with the water in the digester to form explosive hydrogen. Aluminum nitride present combines with the water to form poisonous and flammable ammonia. Aluminum carbide in the dross can result in explosive methane, while contained aluinum phosphides react to yield the toxic and flammable phosphine. In addition, aluminum sulfide in the dross produces flammable and toxic hydrogen sulfide.
Furthermore, not only do the substances formed by the reactions mentioned present a hazard in the workplace and risk the contravention of laws governing workplace conditions, but some of the aluminum present is coverted to unwanted aluiminum oxide, Al.sub.2 O.sub.3, representing an economic loss to operators of the process.
It has been found, however, that where the hydrogen-ion concentration, pH, of the digestion mixture is kept below about 8, the undesirable reactions described are suppressed. While the magnesium chloride, also present as a contaminant in the dross, helps to maintain the pH in the digester below the value mentioned, unfortunately it has a tendency to precipitate from solution as magnesium hydroxide during digestion, a physical state in which it is unable to function as a pH control.
Prior to the invention described herein, introduction of a recycle stream having a substantial magnesium chloride content has not previously been used to control pH in a dross digestion mixture. This is not surprising, given the fact that the presence of magnesium chloride, particularly that resulting from the smelting of used aluiminum beverage cans whcih contain more than 1% by weight of magnesium, presents a problem in dross processing, especially when a wet dross reclamation process is employed.
In this regard, while ordinary aluminum scrap contains only about 0.1%, by weight, of magnesium, beverage cans, which require stiffened lids, necessitate the use of a magnesium-containing aluminum alloy, i.e., one including from about 1.5% to about 5.6%, by weight of magnesium. Furthermore, the magnesium chloride present in the portion of the dross solubilized during the digestion produces a boiling point elevation and crystallizes at a significantly higher temperature than does the sodium chloride and potassium chloride present. Consequently, in that part of the process in which the metal flux salts are recovered by evaporative crystallization, the temperature of the evaporator gradually rises as the magnesium chloride in the crystallizer liquor becomes more concentrated, the rise in temperature eventually causing disruption of the crystallization operation.
pH control in the digester achieved through the incorporation of magnesium chloride extraneous to the recovery system is undesirable for the reason that it increases the overall amount of magnesium contained in the unrecoverable material derived from the process. Such material must eventually be disposed of, commonly in a landfill, and although this primarily non-metallic product, NMP, is placed in plastic film-lined, protected landfills, some water eventually penetrates the liners and can react with the magnesium present, resulting in an exothermic reaction which melts the plastic. The balance of the NMP is thereafter readily accessible to water, which leaches it, the resulting solution being free to enter the environment, particularly the ground water, where it results in unacceptably high chloride levels.